The present invention relates to a polymeric optical waveguide substrate used, in particular, in an optical module used for optical communication as well as a method for the preparation of such a polymeric optical waveguide substrate.
There have recently been investigated various optical parts, which make use of an optical waveguide of quartz for the purpose of making optical parts used for optical communication highly functional, of reducing the size thereof and of saving the production cost and there have been known, for instance, an optical splitter and an arrayed-waveguide grating multiplexer/demultiplexer, which have already been put on the market. Moreover, a small and low cost transceiver module has already been realized by hybrid-packaging semiconductor elements such as a laser diode or a photodiode on a quartz optical waveguide substrate. On the other hand, there have been investigated a variety of polymers as materials for constructing waveguides in addition to quartz. A polymeric waveguide may be formed by the spin coating method and therefore, a waveguide substrate can be prepared in a high production efficiency and at a low cost, as compared with the quartz waveguide. For this reason, an optical module may be realized at a low cost, by the hybrid-packaging technique in which semiconductor elements are mounted on a polymeric waveguide substrate. An example of a polymeric waveguide substrate provided thereon with a semiconductor element such as a laser diode or a photodiode is disclosed in, for instance, The Technical Report of The Proceeding of The Institute of Electronics Information and Communication Engineers, 99-24, pp. 7-12 (August, 1999).
In the article, a polymeric optical waveguide substrate and an optical module are prepared according to the following preparation process: (1) Electrodes are formed on a silicon substrate provided thereon with an oxide layer and a polymeric waveguide is then applied onto the substrate by the spin coating method; (2) the polymeric waveguide on the region of the substrate in which elements are formed is completely removed by the dry etching technique using oxygen gas to thus expose the electrodes; and (3) a laser diode and/or a photodiode are mounted on the electrodes using a solder.
However, the following problems arise in the removal of the polymer layer by the dry etching step (2) in the foregoing preparation process. First of all, the polymer layer to be removed is quite thick on the order of 20 to 30 xcexcm, this step accordingly requires a very long etching time of not less than one hour and therefore, this method is insufficient in mass production ability and cost-saving. Moreover, not only the mask used in the dry etching step should have a high selectivity ratio, but also the polymer layer should be removed by a technique, which never adversely affects the electrodes, the oxide layer and the polymer layer exposed after the etching step. This makes the selection of materials for such a mask very difficult. In addition, if the polymer layer is highly etched to such a high depth, the etched surface is roughened because of the presence of, for instance, foreign body derived from the sputtered mask and/or the chamber, the foreign body remain on the exposed electrodes or unevenness is formed thereon and this may interfere with the welding of semiconductor elements to the electrodes through soldering.
Accordingly, it is an object of the present invention to provide a novel polymeric optical waveguide substrate, which permits the solution of the foregoing problems associated with the conventional techniques.
It is another object of the present invention to provide a method for the preparation of such a polymeric optical waveguide substrate.
The inventors of this invention have conducted various studies to solve the foregoing problems, and have found that the foregoing object of the present invention can be accomplished by a method comprising the steps of (1) forming an electrode on the region of a substrate on which a semiconductor element is to be mounted, (2) applying an adhesive layer capable of improving the adhesion between a polymer layer for a polymeric optical waveguide and the substrate only onto the region of the substrate on which the polymeric waveguide is to be formed, (3) forming the polymer layer for a polymeric waveguide on the whole surface of the substrate, (4) cutting the polymer layer at the boundary between the element mounting region and the waveguide region and (5) removing the polymer layer on the element mounting region to expose the electrode. They have thus completed the present invention.
According to an aspect of the present invention, there is provided a polymeric optical waveguide substrate, which comprises a first region provided thereon with an optical waveguide of a polymer layer and a second region free of any optical waveguide, the first and second regions being present on the same substrate, wherein it comprises an adhesive layer between the lowermost polymer layer constituting the optical waveguide in the first region and the substrate, for improving the adhesion or adhesive properties of them and wherein it has a groove intruding into the substrate at the boundary between the first and second regions.
According to another aspect of the present invention, there is provided a method for the preparation of a polymeric optical waveguide substrate, which comprises a first region provided thereon with an optical waveguide of a polymer layer and a second region free of any optical waveguide, the first and second regions being present on the same substrate, wherein the method comprises the steps of applying an adhesive layer onto the first region for the improvement of the adhesion or adhesive properties of the substrate and the polymer layer for forming an optical waveguide on the surface of the substrate, while the second region is free of any adhesive layer; applying a polymer onto the whole surface of the substrate to give the polymer layer for forming an optical waveguide, after the formation of the adhesive layer on the first region; cutting the polymer layer at the boundary between the first and second regions; and then peeling off or removing the polymer layer on the second region.
In preferred embodiments of the polymeric optical waveguide substrate according to the present invention, it has electrodes on the second region for mounting semiconductor elements; the substrate is a member selected from the group consisting of a silicon substrate, a silicon substrate provided thereon with a silicon oxide layer, a glass substrate and a ceramic substrate; the lowermost layer of the polymer layer constituting the optical waveguide comprises a fluorine-containing polymer and more preferably a fluorinated polyimide and this makes, easy, the removal of the polymer layer on the region on which semiconductor elements are to be mounted; the polymer layer constituting the optical waveguide is formed from a fluorinated polyimide; and the adhesive layer comprises at least one member selected from the group consisting of polyimide-silicone resins, fluorine-free polyimide resins, organic aluminum compounds, organic zirconium compounds and organic titanium compounds.
In preferred embodiments of the method for the preparation of the foregoing polymeric optical waveguide substrate according to the present invention, the electrode is formed on the substrate prior to the formation of the adhesive layer; a dicing device is used for the cutting of the polymer layer and the bottom of the groove thus formed intrudes into the substrate; the substrate is a member selected from the group consisting of a silicon substrate, a silicon substrate provided thereon with a silicon oxide layer, a glass substrate and a ceramic substrate; the lowermost layer of the polymer layer constituting the optical waveguide comprises a fluorine-containing polymer and more preferably a fluorinated polyimide and this makes, easy, the removal of the polymer layer on the region on which semiconductor elements are to be mounted; the polymer layer constituting the optical waveguide is formed from a fluorinated polyimide; and the adhesive layer comprises at least one member selected from the group consisting of polyimide-silicone resins, fluorine-free polyimide resins, organic aluminum compounds, organic zirconium compounds and organic titanium compounds.